(1) Field of the Invention
The present invention relates to a fabrication method used for semiconductor devices, and more specifically to an optimized process for creating reliable, metal filled contact holes, used to connect an active device region to an overlying metallization.
(2) Description of Prior Art
The trend in the semiconductor industry to micro-miniaturazationg enabling the manufacture of higher performing silicon devices, while still maintaining or reducing the cost of semiconductor chips, has created increased reliability demands. Micro-miniaturazation has allowed specific device images, in the sub-micron range, to be routinely fabricated. However when metal filled contact holes, used to connect an overlying metallization to an underlying silicon device region, are fabricated using contact holes with diameters in the 0.50 .mu.M range, severe restrictions on the choice of metal fill exist. The increased current density, now present with smaller diameter contact holes, discourage the use of aluminum based metallurgies for contact fills. The poor electromigration characteristics of this metallurgy presents reliability concerns for advanced designs, using smaller contact hole shapes.
The industry has attempted to solve the current carrying, or reliability situation, by migrating to tungsten or tungsten silicide to fill the small diameter contact holes. The improved electromigration resistance of tungsten type metallzations has resolved the reliability concerns arising with aluminum metallizations. Tungsten is easily deposited and patterned, with acceptable conductivity, and thus has found use in the semiconductor industry as a metal fill, or plug, for small diameter contact holes. However the use of tungsten initially requires a deposition of an adhesive layer, followed by a deposition of a barrier layer, prior to the deposition of the tungsten fill. The adhesive layer, titanium in most cases, is used to provide a stronger bond between the metal fill and the sides of the silicon oxide contact hole, then would have existed if tungsten was used directly in the contact. Titanium, at the bottom of the contact hole where it interfaces silicon, provides an excellent ohmic contact when subjected to elevated temperatures, forming a titanium disilicide layer. The barrier layer, usually titanium nitride, is used to protect the titanium layer from the reaction products generated during the tungsten fill deposition. The tungsten fill is accomplished via low pressure chemical vapor deposition, (LFCVD), using tungsten hexafluoride as source material. Free fluorine ions can attack the exposed titanium adhesive layer during the initial stages of tungsten deposition, if the barrier layer were not employed. This process has been described by Chen, et al, in U.S. Pat. No. 5,286,675. However that process does not account for the poor step coverage or poor conformality of the sputtered titanium nitride barrier layer, resulting in the exposure of underlying titanium to the deleterious attack of fluoride ions produced during the tungsten deposition. The fluoride attack of the exposed titanium layer, through defects in titanium nitride, can lead to ultimate loss of adhesion of the tungsten fill. Another phenomena, the volatile fluoride gas, penetrating along the titanium-titanium interface, at points of poor titanium nitride coverage, can ultimately result in an enhanced peeling, or volcano effect, causing a lifting of the composite layers from the sides of the hole.
This invention will describe a process in which a titanium-titanium nitride composite layer is used with LPCVD tungsten filled contact holes. However this process will incorporate unique annealing steps, needed to produce more effective titanium nitride layers, along with protecting exposed titanium in areas of poor titanium nitride coverage, such that tungsten fills can be accomplished without the occurrence of metal peeling or other yield degrading phenomena.